1. Field of the Invention
The present invention relates to a condensate treatment conducted in a condensate demineralization apparatus of a nuclear power plant, and relates particularly to a method and apparatus for condensate demineralization that enable a high-purity treated water, having a low concentration of sulfate ions derived from organic impurities eluted from a cation exchange resin, to be obtained in a stable manner over a long period of time.
2. Description of Related Art
In a nuclear power plant, following electric power generation using the steam generated within the nuclear reactor or steam generator, the steam is cooled using seawater, and the resulting condensate is then treated with ion exchange resins in a condensate demineralization apparatus, before being fed back into the nuclear reactor or steam generator. This condensate may include seawater components that have seeped into the condensate system, suspended corrosion products generated from plant structural materials and composed mainly of iron oxides (hereafter referred to as “crud”), and ionic impurities. In order to remove these impurities to obtain a high-purity treated water, the nuclear power plant is provided with a condensate demineralization apparatus that uses ion exchange resins to perform a demineralization treatment of the condensate. The ion exchange resins used in the condensate demineralization apparatus include a cation exchange resin that adsorbs cations, and an anion exchange resin that adsorbs anions, and these resins are typically used in combination.
In this condensate demineralization apparatus, in those cases where a cation exchange resin and an anion exchange resin are used in combination, typical examples of the resin combination include combinations of a gel-type cation exchange resin and a gel-type anion exchange resin, and combinations of a porous cation exchange resin and a porous anion exchange resin. Generally, gel-type resins have low osmotic resistance, whereas porous resins have low abrasion resistance, and in consideration of these drawbacks, gel-type resins are typically used in the condensate demineralization apparatus of plants where backwashing regeneration is conducted frequently, whereas porous resins are used in those plants where chemical regeneration is conducted frequently. Porous resins have particularly low abrasion resistance, and therefore during transfer within the condensate demineralization apparatus from the demineralization tower that houses the ion exchange resin bed to the regeneration tower that performs regeneration of the ion exchange resin, contact between resin particles or between resin particles and metal materials may cause damage to the surface of the resin particles or even fracture of the resin particles. Accordingly, in plants such as BWR nuclear power plants where backwashing is used to strip the crud adhered to the surface of the cation exchange resin, a combination of a gel-type cation exchange resin and a gel-type anion exchange resin is typically used due to the more favorable abrasion resistance of these exchange resins.
Moreover, in a porous resin, the resin matrix structure is more dense than that of a gel-type resin, and therefore the rate of diffusion of an adsorbed ion into the particle interior is slower than that observed for a gel-type resin, meaning porous resins tend to exhibit inferior performance in terms of reaction rate and regeneration efficiency. As a result, in those cases where a porous resin is used in a condensate demineralization apparatus, the apparatus design must take these properties of porous resins into consideration, for example by providing an increased regeneration level (namely, the amount of chemical used).
The ion exchange resin used in the condensate demineralization apparatus of a nuclear power plant exhibits a superior removal capability for ionic components such as seawater components typified by NaCl introduced from the upstream side of the resin, but a problem arises in that organic impurities (hereafter abbreviated as “TOC”) composed mainly of polystyrenesulfonic acids tend to be eluted from the cation exchange resin. If this TOC is carried into the nuclear reactor or steam generator, then sulfate ions are generated, which causes a deterioration in the water quality within the nuclear reactor or the steam generator.
Accordingly, in order to raise the nuclear reactor or steam generator water quality to a higher level of purity, the amount of leaked TOC eluted from the demineralization tower filled with the ion exchange resins must be minimized.
Examples of methods that have been proposed to address this problem include a method disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 11-352283, in which a strongly acidic gel-type cation exchange resin is used that has a cross-linking degree of 12 to 16% that is considerably higher than the more typically used cross-linking degree of 8 to 10%, a method disclosed in Japanese Unexamined Patent Application, First Publication No. 2001-314855, in which the anion exchange resin is positioned as the lower layer of the ion exchange resin bed so as to adsorb the TOC eluted from the cation exchange resin, and a method disclosed in Unexamined Patent Application, First Publication No. Hei 8-224579, in which a mixed bed is formed using a strongly acidic gel-type cation exchange resin and a porous anion exchange resin having a Gaussian particle size distribution.